I like the Afterlife title. We have yet to see if that's actually the real title, but I don't mind it.
Calling it Ghostbusters III:[insert chapter title here]
would be a mistake. Ghostbusters II came thirty years ago. Three decades is way too long of a gap to slap a sequential number to a title and have the general public care -- especially when the third movie released in 2016 had nothing to do with the first two. It would just be confusing. We hardcore fans know the difference. The population at large won't.
type "Ghostbusters III" into Google Chrome and you see this
Even though this will be a continuation of the classic universe, the three title carries too much baggage that would lead people to believe it's entirely about the OGBs. We know it won't be, even if they have roles. Better to prime the audience to expect a different direction even if we see familiar faces.
Besides, Ghostbusters III sounds like a skit from the Critic
the fact that I'm using a pop culture reference from 1994 should tell you long the title Ghostbusters III has been a punchline